Open your Game’s uproject file in a text editor. In your case, I believe this will be:
PROJECTROOT/PunchBomb/PunchBomb.uproject
Inside of that file, look for a “Plugins” JSON array. You’ll need to add the OnlineSubsysteamSteam plugin to this array. This will look something like this:
I ended up having to comment out all steam related stuff from my C++ documents and turn on the new steam online system plugin in 4.13 - problem solved!